Method for electrostatic coating of a paper web

ABSTRACT

A method for preliminary treatment of particles of a powder in a dry surface treatment process before applying the powder particles on a surface of a substrate by utilizing an electric field created by electrodes. The Electrodes are located at opposite sides of the substrate in such a way that at least one first electrode is located at the side of the substrate to be coated, and at least one second electrode is located at the opposites side of the substrate. The particles of the powder are pre-charged before bringing them into the electric field.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national stage application of Internationalapplication No. PCT/FI03/00180, filed Mar. 11, 2003 and claims priorityon Finnish Application No. 20020479, Filed Mar. 14, 2002, and FinnishApplication No. 20021253, Filed Jun. 26, 2002.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to a method for a preliminary treatment ofparticles of a powder in a dry surface treatment process before applyingthe powder particles on a surface of a substrate by utilizing anelectric field created by electrodes, which are located at oppositesides of the substrate in such a way that at least one first electrodeis located at the side of the substrate to be coated, and at least onesecond electrode is located at the opposite side of the substrate.

The dry surface treatment process of different substrates, such aspaper, board, plastic, or metallic substrates, comprises dry powderapplication followed by a finishing step, for example thermomechanicalfixing. The application of the powder utilizes an electric field totransfer the powder particles to the surface of the substrate and toenable an electrostatic adhesion prior to the finishing. Both the finaladhesion and the surface smoothening of the dry powder are executedsimultaneously through thermomechanical treatment or another suitabletreatment. The powder, which is used, may be a coating compositioncomprising inorganic particles and binder particles, or a film formingmaterial, which can be finished so that a pinhole-free film layer isformed.

In a dry surface treatment process, the charging of the powder has anessential role. If some inadequacies relating an amount of the chargedparticles, or a level of charging of a particle occur, it has an effecton efficiency and a cleanliness of the process. If the particles of thedry powder do not adhere properly to a substrate it causes an unevenpowder layer on the substrate, dusting, material losses, and possiblyharmful deposits.

SUMMARY OF THE INVENTION

The method of the invention is an enhancement to the dry surfacetreatment process, and it diminishes the above-mentioned problems of thedry surface treatment process. The method is characterized in that theparticles of the powder are pre-charged before bringing them into theelectric field.

The method of the invention makes the efficiency of the dry surfacetreatment process better because the dry powder places itself on thesubstrate properly, and no material losses occur. As a consequence, alsoa coating layer of a higher quality is achieved. A clean process withoutdusting is also attained.

In a dry surface treatment process, an electric field is created betweenelectrodes, which are in different potentials. A substrate to be coatedis between the electrodes. At least one of the electrodes may be acorona charging electrode which charges surrounding gas. The charged gasatoms, molecules or molecule groups attach to particles of the coatingpowder, thus giving a charge to the particle.

A force according to the equation F=qĒ, where Ē is an electric field, Fis the force, and q is a charge, has an influence on a charge q in anelectric field E. The force tends to convey the charge in the electricfield, and in a stationary electric field only a position of the chargeis meaningful. When a potential of the electric field is known, thestrength of the electric field in a certain position is derived from thefollowing equation:

${\overset{\_}{E} = {{- \frac{\partial V}{\partial n}}{\overset{\_}{u}}_{n}}},{where}$$\overset{\_}{E}\mspace{14mu}\text{is~~the~~strength~~of~~the~~electric~~field,}$V  is  the  potential  of  the  electric  field,  and${\overset{\_}{u}}_{n}\mspace{14mu}\text{is~~the~~unit~~vector~~of~~the~~perpendicular~~of~~the~~plane.}$

The strength of the electric field can also be evaluated by the electriccharge density when the relation between the electric flux density andthe strength of the electric field is taken into consideration. Theequation below is Gauss's law.

${{\frac{\partial E_{x}}{\partial x} + \frac{\partial E_{y}}{\partial y} + \frac{\partial E_{z}}{\partial z}} = \frac{\rho}{ɛ}},{where}$E_(x, y, z)  are  the  dimensional  strengths  of  the  electric  field,ρ  is  a  local  electric  charge  density,  andɛ  is  the  dielectric  constant  of  the  examined  space.

As seen from the equation above, in a stationary electric field thecharges create the field, and the distribution and the magnitude of thecharges determine the strength of the field in different positions.

If a conductive particle (radius a, charge q) is exposed to a uniformelectric field E₀ in a unipolaric ion concentration N₀, the electricfield at the particle is formed from two components, namely an electricfield created by the particle itself due to its own charge, and an outerelectric field which is changed by the charge of the particle. Thisfield is described by an equation

$E = {{3E_{0}\cos\;\theta} - {\frac{q}{4\pi\; ɛ_{0}a^{2}}\mspace{14mu}{where}}}$E  is  the  resultant  electric  field,θ  Is  the  incidence  angle  of  the  electric  field  focused  to  a  particle, ɛ₀  is  the  dielectric  constant  of  the  free  space,a  is  the  diameter  of  the  particle,  andq  is  the  charge  of  the  particle.

The term 3E₀cos θ describes the change of the electric field as aconsequence of the presence of the conductive particle. E₀ is theundisturbed field. The charging of the particles is great, and it isrestricted only by the ions conveyed onto the particle by the electricfield. The change of the charge is defined as a stream, and it can bedescribed by the equation

$\frac{\mathbb{d}q}{\mathbb{d}t} = {N_{0}{\mathbb{e}}\; b{\int_{\theta}^{\theta_{0}}{\left( {{3E_{0}\cos\;\theta} - \frac{q}{4\pi\; ɛ_{0}a^{2}}} \right){\mathbb{d}A}\mspace{14mu}{where}}}}$q  is  the  charge  of  the  particle, b  is  the  mobility  of  ion,e  is  the  charge  of  the  electron,θ₀  is  the  critical  angle  of  to  the  particle  streaming  charge,dA  is  the  area  of  the  component = 2π a²sin  θ d θ, and t  is  time

The above mentioned equation shows that the charge continues to streamto the particle until the field created by the particle and the outerfield are balanced out. When a saturation charge is known, the level ofcharging can be derived from the equation

${q(t)} = {q_{s}\frac{1}{1 + {\tau/t}}}$where τ=4ε₀ /N ₀ eb

For conductive materials, the saturation charge is q_(s)=12πa²ε₀E₀. Fornon-conductive materials, to the equation shall be added 3κ/(κ+2).

The diffusion charge has also an effect on the particle. As a functionof time, the diffusion charge can be expressed by the equation

${q(t)} = {\frac{akT}{e}1{n\left( {1 + \frac{\pi\;{avN}_{0}e^{2}t}{kT}} \right)}\mspace{14mu}{where}}$k  is  Bolzman’s  constant, T  is  the  temperature  (K.),e  is  the  charge  of  the  electron,v  is  the  thermal  velocity  of  ions  (rms),N₀  is  the  average  amount  of  molecules  in  a  certain  volume,   and t  is  time.

As can be concluded from the above mentioned equations, time is animportant factor in charging of particles.

The pre-charging of the particles of the coating powder can be madeeither when the particles are brought at the final electric field orbefore bringing them into the final electric field. The aim of thepre-charging is to obtain a longer charging period compared to theprocess having only one charging step. The benefits of the longercharging period are a more homogenous charging level and a greater forceof the electric field having influence on the particle.

The first embodiment of the invention is to pre-charge the particles ofthe coating powder when they are about to arrive into the final electricfield. The pre-charging process is conducted in such a way that at leastone charging electrode comprising a feeding nozzle is located fartheraway from the substrate to be coated than other charging electrodes. Thedry powder is led to the charging electrode, and particles of the drypowder are charged by the charging electrode. After that the pre-chargedparticles enter to the final electric field formed by the other chargingelectrodes, for example corona charging electrodes, on the same side ofthe substrate and a grounding electrode, or an electrode having anopposite sign on the opposite side of the substrate. The pre-chargedparticles are blown towards a substrate to be coated. The substrate ispreferably in a web form. The grounding electrode can be a stationaryplaty electrode, or it can be a roll rotating about its axis. Therotating roll is a preferred choice.

The second embodiment of the invention is to pre-charge the particles ofthe coating powder in another electric field(s) before the finalelectric field. In this embodiment, a dry powder is led first to aseparate electric field and after that to the final electric field.Particles of the dry powder are pre-charged in a charging unitcomprising a corona charging electrode, an electrode having a differentpotential compared to the corona charging electrode (e.g. a groundingelectrode, an electrode in a lower or opposite potential), and a feedingnozzle.

Particles may also be charged by triboelectric charging, for examplecharging the particles by a friction between the particles, and walls ofa transfer pipe, or a storage bin. After that the particles enter toanother charging unit, which conducts the final charging of theparticles. The final electric field is formed by electrodes at appositesides of the substrate. The electrodes can be corona charging electrodesand a grounding electrode; other suitable electrodes and a groundingelectrode; or electrodes being of different potentials at opposite sidesof the substrate. The pre-charged particles are blown towards asubstrate to be coated through a nozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described by means of figures.

FIG. 1 shows the first embodiment of the invention.

FIG. 2 shows the second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 1, a dry powder is led to a charging electrode 1comprising a feeding nozzle. Particles of the dry powder are charged bythe charging electrode 1. The charging electrode is located farther fromthe substrate 4 than other electrodes 2 so that the particles arepre-charged when they enter to the final electric field formed by thecorona charging electrodes 1,2 and a grounding electrode 3. Thepre-charged particles are blown towards a substrate 4 to be coated. Thesubstrate 4 is preferably in a web form. The grounding electrode 3 canbe a stationary platy electrode, or it can be a roll rotating about itsaxis. The rotating roll is a preferred choice.

According to FIG. 2, a dry powder is led to a first electric field andafter that to a second electric field. Particles of the dry powder arecharged in a charging unit 7 comprising a corona charging electrode 6, agrounding electrode 5, and a feeding nozzle 8. The particles arepre-charged in the first electric field created in the charging unit 7before entering to the second electric field formed by the coronacharging electrodes 2 and a grounding electrode 3. The pre-chargedparticles are blown towards a substrate 4 to be coated. As in theembodiment shown in FIG. 1, the remarks concerning the form of thesubstrate 4 and the preferred grounding electrode 3 are also valid inthis embodiment.

The invention is not restricted to the description above, but theinvention may vary within the scope of the claims.

1. A method for coating a paper or board web in a dry surface treatmentprocess, wherein motion of the paper or board web defines an upstreamand a downstream direction, comprising the steps of: pre-chargingparticles of a dry powder within a charging unit by causing the drypowder to move between an electrode producing corona charging within theunit and an electrode at a lower or opposite potential within thecharging unit to form pre-charged particles; supplying the pre-chargedparticles from the charging unit to a feeding nozzle which forms anelectrode and blowing the pre-charged particles from the feeding nozzletoward the paper or board web, the feeding nozzle being positionedbetween an upstream electrode producing a corona discharge, the upstreamelectrode positioned outside of the charging unit and laterally spacedin the upstream direction from the feeding nozzle, and a downstreamelectrode producing a corona discharge, the downstream electrodepositioned outside of the charging unit and laterally spaced in thedownstream direction from the feeding nozzle, wherein the feeding nozzleis spaced further from the paper or board web than the upstreamelectrode and the downstream electrode; wherein the paper or board webis backed by a grounding electrode at a potential which is lower than oropposite to the potentials of the feeding nozzle, the upstream electrodeand the downstream electrode.
 2. The method of claim 1 wherein thegrounding electrode is a rotatable roll.
 3. The method of claim 1wherein the grounding electrode is a stationary platy electrode.
 4. Amethod for coating a paper or board web in a dry surface treatmentprocess, wherein motion of the paper or board web defines an upstreamand a downstream direction, comprising the steps of: pre-chargingparticles of a dry powder by causing the dry powder to move along thewalls of a transfer pipe to charge the particles by triboelectriccharging; supplying the pre-charged particles to a feeding nozzleforming an electrode and blowing the pre-charged particles from thefeeding nozzle toward the paper or board web, the feeding nozzle beingpositioned between an upstream electrode producing a corona discharge,the upstream electrode positioned laterally spaced in the upstreamdirection from the feeding nozzle, and a downstream electrode producinga corona discharge, the downstream electrode positioned laterally spacedin the downstream direction from the feeding nozzle, wherein the feedingnozzle is spaced further from the paper or board web than the upstreamelectrode and the downstream electrode; wherein the paper or board webis backed by a grounding electrode at a potential which is lower than oropposite to the potentials of the feeding nozzle, the upstreamelectrode, and the downstream electrode.
 5. The method of claim 4wherein the grounding electrode is a rotatable roll.
 6. The method ofclaim 4 wherein the grounding electrode is a stationary platy electrode.7. A method for coating a moving web using a dry surface treatmentprocess wherein the movement of the web defines an upstream directionand a downstream direction, comprising the steps of: pre-chargingparticles of a dry powder within a charging unit by causing the drypowder to move between an electrode producing corona charging within theunit and an electrode at a lower or opposite potential within thecharging unit to form pre-charged particles; coating the web with acoating layer by supplying the pre-charged particles from the chargingunit to a feeding nozzle which forms an electrode and blowing thepre-charged particles from the feeding nozzle toward the web, thefeeding nozzle being positioned between an upstream electrode producinga corona discharge, the upstream electrode positioned outside of thecharging unit and laterally spaced in the upstream direction from thefeeding nozzle and a downstream electrode producing a corona discharge,the downstream electrode positioned outside of the charging unit andlaterally spaced in the downstream direction from the feeding nozzle,wherein the feeding nozzle is spaced further from the web than theupstream electrode and the downstream electrode; and wherein the web isbacked by a grounding electrode at a potential which is lower than oropposite to the potentials of the feeding nozzle, the upstream and thedownstream electrodes.
 8. The method of claim 7 wherein the groundingelectrode is a rotatable roll.
 9. The method of claim 7 wherein thegrounding electrode is a stationary platy electrode.
 10. A method forcoating a moving web using a dry surface treatment process wherein themovement of the paper or board web defines an upstream direction and adownstream direction, comprising the steps of: pre-charging particles ofa dry powder by causing the dry powder to move along the walls of atransfer pipe to charge the particles by triboelectric charging;supplying the pre-charged particles to a feeding nozzle forming anelectrode and blowing the pre-charged particles from the feeding nozzletoward the web, the feeding nozzle being positioned between an upstreamelectrode producing a corona discharge, the upstream electrodepositioned laterally spaced in the upstream direction from the feedingnozzle, and a downstream electrode producing a corona discharge, thedownstream electrode positioned laterally spaced in the downstreamdirection from the feeding nozzle, wherein the feeding nozzle is spacedfurther from the paper or board web than the upstream electrode and thedownstream electrode; wherein the paper or board web is backed by agrounding electrode at a potential which is lower than or opposite tothe potentials of the feeding nozzle, the upstream electrode, and thedownstream electrode.
 11. The method of claim 10 wherein the groundingelectrode is a rotatable roll.
 12. The method of claim 10 wherein thegrounding electrode is a stationary platy electrode.